\documentclass[tikz]{standalone}
\usepackage{tikz}
\usetikzlibrary{graphs}
\begin{document}
\begin{tikzpicture}
    \graph {
        "$x_1$" -> "$x_2$"[red] -> "$x_3,x_4$";
        "$x_1$" ->[bend left] "$x_3,x_4$";
	};
\end{tikzpicture}
\begin{tikzpicture}
    \graph  {
     a -> {
        b -> c,
        d -> e
     } -> f
    };
\end{tikzpicture}
\end{document}